1. Field of the Invention
This invention relates to an optical system and an original reading apparatus, and particularly is adapted to highly accurately read image information on the surface of an original by a reading element (solid state image pickup element) such as a line sensor by the use of an original reading lens having a diffraction grating.
2. Related Background Art
There have heretofore been proposed various original reading apparatuses adapted to reduce and image the image information on the surface of an original such as a document or literature on the surface of a line sensor (CCD) by an original reading lens, and read the image information as electronic information by a signal from the line sensor.
In this case, as the original reading lens, a lens system is desired which is compact as a whole and moreover can easily obtain relatively high optical performance. As an original reading lens satisfying such a desire, there is, for example, a symmetrical type lens system comprising a plurality of lenses disposed symmetrically about a stop. At this symmetrical type lens system, there is, for example, a Gaussian type lens of four-unit six-lens construction.
Generally the symmetrical type lens system has the feature that images of particularly high resolving power and high quality can be obtained easily.
When the image information on the surface of the original is to be reduced and imaged on the surface of the line sensor by the original reading lens, and the image information is to be read as electronic information by the signal from the line sensor, it becomes important for the whole of the surface of the original to be imaged with high resolving power on the surface of the line sensor.
As the original reading lens, for example, a Tesser type one of three-unit four-lens construction is relatively sufficiently well corrected in various aberrations such as spherical aberration, field curvature aberration and distortion aberration.
However, there has been a tendency for astigmatism at a medium angle of view and off-axis coma to remain high. Therefore, the original reading lens of the Tesser type is used in an original reading apparatus having relatively low resolution and a narrow angle of view.
Also, the original reading lens of the Tesser type is corrected to a certain level in chromatic aberration, particularly on-axis chromatic aberration, but could not always be said to be sufficiently satisfactory as a color image reading lens system.
Particularly it is over corrected at the short wavelength side of the visible wavelength area of the on-axis chromatic aberration, and is conversely under-corrected at the long wavelength side, and it has been difficult to well correct the on-axis chromatic aberration in a wide range of the visible wavelength area and such original reading lens has had residual chromatic aberration (secondary spectrum). This has led to the problem that when such an original reading lens is used, for example, in a color image reading apparatus such as an image scanner, the focus positions for R (red), G (green) and B (blue) color lights somewhat differ from one another and the deterioration of read images occurs.
On the other hand, a Gaussian type original reading lens of four-unit six-lens construction is sufficiently corrected in various aberrations such as spherical aberration, coma and field curvature aberration.
Regarding chromatic aberration, however, it has residual chromatic aberration (secondary spectrum) like the aforedescribed Tesser type original reading lens of three-unit four-lens construction. This has led to the problem that the focus positions for R, G and B color lights differ from one another and the deterioration of read images occurs.
To correct this residual chromatic aberration for a wide band wavelength, it is necessary to use glass having an abnormal portion dispersing property which is also small in the variation in refractive index for the wide band wavelength (abnormal dispersion glass), but there has been the problem that the glass of this type is generally expensive and difficult to work.
Also, there has been proposed an original reading lens using a diffraction grating for reducing chromatic aberration. This original reading lens, however, has suffered from the problem that the deterioration of read images occurs due to flare lights of the other orders than the design order. As a technique for reducing such flare lights, there is the technique of laminating two kinds of resins 105 and 106 differing in dispersing characteristic on one of substrates 104 as shown, for example, in FIGS. 14 and 15 of the accompanying drawings. However, the type of FIG. 14 in which the grating depths d1 and d2 differ from each other has suffered from the problem that manufacture is difficult, and the type of FIG. 15 has suffered from the problem that the grating depth d is as great as 80-90 xcexcm and rays of light with an angle of view are reflected by the portions of the walls of the grating.
It is an object of the present invention to provide an original reading lens which can well correct particularly on-axis chromatic aberration and chromatic aberration of magnification of various magnifications over a wide band by utilizing a laminated type optical element (laminated type diffraction grating) easy to manufacture, and highly accurately read image information as electronic information, and an original reading apparatus using the same.
It is another object of the present invention to provide an original reading lens of four-unit four-lens construction comprising four units and four lenses disposed substantially symmetrically about a stop in which the lens construction can be set appropriately and which can reduce and image the image information of the entire surface of an original with high resolving power on the surface of a reading element in spite of the number of constituent lenses being small, and can decrease flare lights of the other orders than the design order of a diffraction grating, and an original reading apparatus using the same.
An optical system according to an embodiment of the present invention is an optical system provided with a lens, a stop and a diffraction grating, characterized in that when in a state wherein the diffraction grating is absent, the value of the coefficient of chromatic aberration of magnification of the wavelength of the long wavelength side relative to the reference wavelength of light is positive, the diffraction grating is disposed more adjacent to an incidence surface side than the stop, and when in the state wherein the diffraction grating is absent, the value of the coefficient of chromatic aberration of magnification of the wavelength of the long wavelength side relative to the reference wavelength of the light is negative, the diffraction grating is disposed more adjacent to an emergence side than the stop.
An optical system according to another embodiment of the present invention is the above-described optical system characterized in that the lens constituting the optical system comprises four or less lenses, and is an optical element in which in the state wherein the diffraction grating is absent, the value of the coefficient of chromatic aberration of magnification of the wavelength of the long wavelength side relative to the reference wavelength of the light is positive, and the diffraction grating is disposed more adjacent to the incidence surface side than the stop.
An optical system according to another embodiment of the present invention is the above-described optical system characterized in that the lens constituting the optical system consists, in succession from the incidence surface side, a meniscus-shaped positive first lens having its convex surface facing the incidence surface side, a meniscus-shaped negative second lens having its convex surface facing the incidence surface side, a stop, a meniscus-shaped negative third lens having its convex surface facing the emergence side, and a meniscus-shaped positive fourth lens having its convex surface facing the emergence side.
An optical system according to another embodiment of the present invention is the above-described optical system characterized in that the diffraction grating is disposed between the second lens and the stop.
An optical system according to another embodiment of the present invention is the above-described optical system characterized in that when the Abbe numbers of the materials of the first lens and the fourth lens are defined as vp1 and vp2, respectively, and the Abbe numbers of the materials of the second lens and the third lens are defined as vm1 and vm2, respectively, the Abbe numbers vp1 and vp2 are 50 or greater and the Abbe numbers vm1 and vm2 are 35 or less.
An original reading apparatus according to an embodiment of the present invention is an original reading apparatus provided with an original reading lens for imaging image information on the surface of an original illuminated by a light source on the surface of a reading element and a stop and a diffraction grating disposed in the original reading lens. The original reading apparatus is characterized in that when in a state wherein the diffraction grating is absent, the value of the coefficient of chromatic aberration of magnification of the wavelength of the long wavelength side relative to the reference wavelength of the light source is positive, the diffraction grating is disposed more adjacent to the surface of the original than the stop. When in the state wherein the diffraction grating is absent, the value of the coefficient of chromatic aberration of magnification of the wavelength of the long wavelength side relative to the reference wavelength of the light source is negative, the diffraction grating is disposed more adjacent to the reading element than the stop.
An original reading apparatus according to another embodiment of the present invention is the above-described original reading apparatus characterized in that the lens constituting the original reading lens comprises four or less lenses, and is a lens in which in the state wherein the diffraction grating is absent, the value of the coefficient of chromatic aberration of magnification of the wavelength of the long wavelength side relative to the reference wavelength of the light source is positive, and the diffraction grating is disposed more adjacent to the surface of the original than the stop.
An original reading apparatus according to another embodiment of the present invention is the above-described original reading apparatus characterized in that the original reading lens consists, in succession from the surface side of the original, a meniscus-shaped positive first lens having its convex surface facing the surface side of the original, a meniscus-shaped negative second lens having its convex surface facing the surface side of the original, a stop, a meniscus-shaped negative third lens having its convex surface facing the reading element side, and a meniscus-shaped positive fourth lens having its convex surface facing the reading element side.
An original reading apparatus according to another embodiment of the present invention is the above-described original reading apparatus characterized in that the diffraction grating is disposed between the second lens and the stop.
An original reading apparatus according to another embodiment of the present invention is the above-described original reading apparatus characterized in that when the Abbe numbers of the materials of the first lens and the fourth lens are defined as vp1 and vp2, respectively, and the Abbe numbers of the materials of the second lens and the third lens are defined as vm1 and vm2, respectively, the Abbe numbers vp1 and vp2 are 50 or greater and the Abbe numbers vm1 and vm2 are 35 or less.
An original reading apparatus according to another embodiment of the present invention is an original reading apparatus provided with an original reading lens for imaging image information on the surface of an original illuminated by a light source on the surface of a reading element. The original reading apparatus is characterized in that the original reading lens has a laminated type optical element in which the diffraction grating surfaces of a first substrate provided with a first optical element having a diffraction grating of a convex shape and a second substrate provided with a second optical element having a diffraction grating of a concave shape are joined together in opposed relationship with each other.
An original reading apparatus according to another embodiment of the present invention is the above-described original reading apparatus characterized in that of distracted lights of respective orders distracted by the diffraction gratings and incident on the reading element, diffracted lights of the other orders than the design order are set so that the quantity of light thereof may be 5% or less relative to the quantity of light of the diffracted light of the design order.
An original reading apparatus according to another embodiment of the present invention is the above-described original reading apparatus characterized in that of diffracted lights of respective orders diffracted by the diffraction gratings and incident on the reading element, diffracted lights of the other orders than design order are set so that the quantity of on-axis light thereof and the quantity of the most off-axis light thereof may be substantially equal to each other.
An original reading apparatus according to another embodiment of the present invention is the above-described original reading apparatus characterized in that when the grating depths of the diffraction gratings of the first and second optical elements are defined as d1 and d2, respectively, the grating depths d1 and d2 are determined from the following expressions:
{n1(xcexa)xe2x88x921}d1xe2x88x92{n2(xcexa)xe2x88x921}d2=mxcexa
{n1(xcexb)xe2x88x921}d1xe2x88x92{n2(xcexb)xe2x88x921}d2=mxcexb
0.40 less than xcexa less than 0.50
0.51 less than xcexb less than 0.62
where
n1: the refractive index of the material of the first optical element;
n2: the refractive index of the material of the second optical element;
xcexa: the first set wavelength (xcexcm);
xcexb: the second set wavelength (xcexcm):
m: 1, 2, 3, . . . .
An original reading apparatus according to another embodiment of the present invention is the above-described original reading apparatus characterized in that when in a state wherein the laminated type optical element is absent, the value of the coefficient of chromatic aberration of magnification of the wavelength of the long wavelength side relative to the reference wavelength of the light source is positive, the laminated type optical element is disposed more adjacent to the surface side of the original than the stop. When in the state wherein the laminated type optical element is absent, the value of the coefficient of chromatic aberration of magnification of the wavelength of the long wavelength side relative to the reference wavelength of the light source is negative, the laminated type optical element is disposed more adjacent to the reading element side than the stop.
An original reading apparatus according to another embodiment is the above-described original reading apparatus characterized in that the lens constituting the original reading lens comprises four or less lenses, and is a lens in which in the state wherein the laminated type optical element is absent, the value of the coefficient of chromatic aberration of magnification of the wavelength of the long wavelength side relative to the reference wavelength of the light source is positive, and the laminated type optical element is disposed more adjacent to the surface side of the original than the stop.